The present proposal is for continuation of a long-standing research program directed at identifying and characterizing the ion exchangers mediating acid-base and NaC1 transport in the proximal tubule. In the proximal tubule, the principal apical membrane pathway for absorption of Na+ and secretion of H+ is Na+-H+ exchange. Cl- absorption across the apical membrane in this nephron segment occurs by several different C1--base exchange processes including CL--formate exchange. C1--oxalate exchange and Cl- -OH- exchange. During the past project period we have used a variety of experimental approaches to demonstrate that NHE3 is the isoform principally responsible for apical membrane Na+-H+ exchange in the proximal tubule. In addition, we have generated and used anti-NHE3 monoclonal antibodies to discover unexpected aspects of the structure of NHE3 including its association in oligomeric complexes with dipeptidyl peptidase IV, and the fact that epitopes within the C-terminal hydrophilic "cytoplasmic" domain are actually exoplasmic. In the next project period we will pursue studies to define the functional consequences of these two novel aspects of the structure of NHE3. In related studies, we have completed the cloning of a new member of the NHE gene family that is expressed in the kidney. We propose to characterize the functional properties of this new NHE isoform and determine whether it is expressed in the proximal tubule and/or other nephron segments. Finally, we have identified a novel anion transporter that is expressed on the apical membrane of proximal tubule cells and mediates C1--formate exchange (CFEX). We plan to characterize the functional properties of CFEX in more detail and will seek to identify additional transporters that may be responsible for apical Cl--base exchange in the proximal tubule. Taken together, the proposed studies should enhance understanding of the mechanisms of Na+, HCO3-- and CI-- transport in the kidney, which is relevant to clinical disorders caused by alterations in renal acid-base balance (eg. nephrolithiasis, osteoporosis) and NaC1 transport (eg. hypertension, congestive heart failure).